Firstly, ensure you understand best practices:
- Watch this video (form Channel 9) : Inside Windows Phone #03: Optimizing Windows Phone Silverlight Applications
- Read this White Paper: Performance Considerations in Applications for Windows Phone
- Read the performance tips on the Windows Phone 7 Development Best Practices Wiki
Secondly, use the available tools to analyse your apps before releasing them:
- Enable the peformance category CODE_ANALYSIS ruleset in Visual Studio (but only if you have Premium or Ultimate edition).
- Try the profiler in Silverlight Spy (4.0) (works with the emulator)
- Try the profiler from EQATEC
Thirdly, add analytics to your app to understand how it's used and any issues users are actually having:
Got any other tips?